The field of this invention relates to the liquid-phase continuous oxidation of pseudocumene to trimellitic acid.
The possibility of using liquid-phase instead of vapor-phase oxidation for the preparation of benzene carboxylic acids was first indicated by the disclosure in U.S. Pat. No. 2,245,528 of the catalysis provided by transition or variable-valence metals, especially cobalt, in temperatures from 100.degree. C. to 320.degree. C. and pressures to maintain the liquid phase of the aliphatic acid. Such catalysis, according to said patent, was advantageously promoted by the use of a ketone, such as methylethyl ketone, or aldehyde, such as acetaldehyde. It has been very difficult to manufacture trimellitic acid from pseudocumene in a continuous process because trimellitic acid (TMLA) is a catalyst deactivator. The usual process is to manufacture trimellitic anhydride from pseudocumene using a batch process. At start-up in a batch process, the concentration of TMLA is zero. As the process continues, concentration of TMLA builds up and deactivates the catalyst at the end of the process. In such prior art processes, the yields often do not exceed eighty-six mole percent and approximately two mole percent of high molecular weight impurities are formed.
It has been discovered that by employing a continuous process for the manufacture of trimellitic acid from pseudocumene, the yields of trimellitic acid are improved to 92 mole percent and the high molecular weight impurities are usually reduced about fifty percent to about one mole percent.
According to U.S. Pat. No. 3,920,735, the Mn-Br and Co-Mn-Br catalyst systems described in U.S. Pat. No. 2,245,528 are improved by the addition of zirconium. However, not mentioned, but illustrated in Tables I, II, and IV in U.S. Pat. No. 3,920,735 is the fact that, when part of the zirconium is added, combustion of the feedstock to carbon dioxide increases.
In the prior art, it has been known to use a staged catalyst addition oxidation of pseudocumene to prepare trimellitic anhydride in a batch method. For example, U.S. Pat. No. 3,683,016 to Darin, et al, teaches a method of staged catalyst addition of the catalyst components of cobalt, cerium and manganese in combination with a source of bromine wherein 10 to 25 percent of the manganese is added initially to the reaction in a first stage together with all the cobalt and approximately one-half of the cerium. The remaining 75 to 90 percent of the manganese together with cerium in an amount substantially equal to the cerium added in the first stage is added to the second stage. The reaction temperature of the first stage is in the range of 360.degree. F. to 390.degree. F. The temperature in the second stage is upward from 400.degree. F., preferably from 400.degree. F. to 435.degree. F. The continuous method taught by Darin requires a series of batch operations wherein each staged addition of catalyst components is added to an oxidation vessel connected in series with the preceding vessel. The semi-continuous method of Darin also requires batch operation by batch addition of catalyst components. Accordingly, Darin, et al, U.S. Pat. No. 3,683,016 teaches batch methods to obtain continuous operation of staged catalyst addition using a cerium component catalyst at high temperatures to oxidize pseudocumene to trimellitic acid. However, in the instant invented continuous process, the oxidation mode is staged wherein pseudocumene is added continuously to the first oxidation stage, catalyst is added continuously during both oxidation stages, a cobalt-manganese-brcmine catalyst or zirconium, cobalt-manganese-bromine catalyst is used, temperature of the first stage is from about 250.degree. F. to about 350.degree. F., temperature in the second stage is from about 350.degree. F. to 450.degree. F. and catalyst is continuously recycled as the oxalate salt to the first oxidation stage.
Also, U.S. Pat. No. 4,398,040, to Kitahara, et al, teaches a method of producing trimellitic acid by liquid-phase air oxidation of pseudocumene in acetic acid in the presence of cobalt, manganese and bromine as catalysts using a very low oxygen partial pressure whereby the reaction is in plural stages, the first stage at a temperature of 110.degree. C. to 170.degree. C. and the second stage at 180.degree. C. to 240.degree. C. In the examples, pressure in the first stage is 3.4 to 3.9 atm. and in the second stage, pressure is 22.3 atm. Reaction temperature is controlled by adjusting reaction pressure. Catalyst required by both stages of the oxidation process is taught as added in the first stage, and preferably not changed, by addition, to change the composition of the catalyst between the first stage of oxidation and the second stage of oxidation. Accordingly, Kitahara, et al, U.S. Pat. No. 4,398,040, teaches a method of staged temperature control by adjusting reaction pressure wherein catalyst addition is preferably not changed by further addition of catalyst after the first stage of oxidation. Catalyst recovery is by dehydrating the mother liquor. However, in the instant invented continuous process, there is taught a continuous method of staged oxidation wherein catalyst is added during both oxidation stages, and recycled, and the oxidation reaction stages are at low pressures. Reaction pressure is not a critical process variable. Additionally, catalyst recovery and recycle is by formation of oxalate salts.